Coil Current Calculation

[steris]

So I'm a Mech E trying to track down causes of failure within an electromechanical system. One part of the system involves a pressure switch that energizes a coil on a contactor. The contactor then powers a set of heaters. The pressure switch and coil both run off of 120V AC. When characterizing the load on the switch, I measured the resistance through the coil (55 Ohms) and calculated the current as I=V/R. However when I measure the current using an ammeter, the current is about 92% lower than the calculated value. The difference is too large to be an issue with RMS voltage vs peak voltage. I feel like I am missing something fundamental. What am I doing wrong?

Thanks in advance!
-Steris

 

[Skogsgurra]

You are using DC resistance to calculate current in an AC coil. It doesn't work that way.

An AC coil has reactance and that reactance ("AC ohms" if you like) keeps current way below what you get when dividing AC voltage by DC resistance.

Your values sound OK. No probs there.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[desertfox]

Hi steris

Skogsgurra is spot on have a look at this site it might help explain reactance.

http://en.wikipedia.org/wiki/Electrical_impedance

desertfox

 

[steris]

Thanks a lot! That makes a lot more sense. I never worked much with AC. I greatly appreciate the help!

 

[desertfox]

Hi steris

your welcome

desertfox